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Lunar and Planetary Science XXXVI (2005) 1172.pdf

MGS MOC: FIRST VIEWS OF AT SUB-METER RESOLUTION FROM ORBIT. M. C. Malin and K. S. Edgett, Malin Space Science Systems, PO Box 910148, San Diego, CA 92191-0148 USA.

Introduction: The (MGS) pitching, rolling, and moving along its orbit, Mars is (MOC) is now capable of rotating underneath it. The pitch and roll are timed to obtaining images with sub-meter per pixel spatial account for the rotation of Mars, as well as the resolution. In such images, the two Mars Exploration desired image resolution and target location. Rovers (MER) can be seen from orbit, 380–400 km Limitations: Opportunities to acquire cPROTO away (Fig. 1a). These images also permit testing of images are limited by spacecraft communication hypotheses regarding a variety of landforms schedules, because the spacecraft cannot and geologic materials. MOC was designed to obtain communicate with Earth during a cPROTO images with spatial resolution as high as ~1.5 m/pixel maneuver, and by spacecraft solar power, because the from its nominal altitude orbit, using spacecraft solar panels cannot point at the Sun when a cPROTO motion to sweep a single line detector array across a is being executed. The size of a cPROTO image is target area [1]. In 2003 and 2004, operators of MOC limited by how much data the MOC can collect and and MGS pioneered an approach that allows MOC to place in its internal buffer, and the selection of a obtain images with ~50 cm/pixel spatial resolution in cPROTO target is limited by atmospheric opacity, the downtrack dimension. Images acquired using the solar illumination of the surface, and protection of cPROTO (compensated Pitch and Roll Targeted MOC’s optical system from direct sunlight. Given Observation) approach, effectively, provide sub- these limitations, only certain parts of Mars are meter resolution views of Mars. accessible to cPROTO imaging at any specific time cPROTO Maneuver: To acquire a cPROTO during the year. Furthermore, even after a target is image, the entire MGS spacecraft is moved in pitch selected, it is possible that the image will miss the and roll directions. A typical image covers an area of target or that other factors (gain/offset selection, about 3 by 3 km. Owing to as-yet unresolved unexpected clouds/dust storm) will contribute to the uncertainty in spacecraft motion, specific targets difficulty in achieving a desired goal. within a 3x3 km area are often missed by more than Initial Results: The first 3 sub-meter images 2 km. In other words, the majority of cPROTO were PROTO (no compensation for Mars rotation). A images do not hit their intended targets; 2–4 total of 69 PROTO and cPROTO images were acquisitions are often needed to cover a feature of acquired and received from May 2003 through interest. During normal operations, MGS keeps its December 2004. A product of MOC science goals nadir deck, on which the MOC is fixed/mounted, and the limitations described above, the majority of pointed toward the planet. To do so, the spacecraft cPROTO images have thus far focused on 6 specific rotates about its Y-axis once per orbit. For cPROTOs, target types: landing sites, gullies, sedimentary rocks, this rotation rate is changed by looking forward a bit fretted terrain valley floor textures, north polar and speeding up the spacecraft to stare at the target residual cap and layered materials, and eolian location while MGS flies over it, before returning to features. Other images examined outflow channel the downward stare. In this way, the apparent landforms, ridged-textured materials associated with forward speed of the spacecraft is reduced, allowing sedimentary rocks, and products of mass movement either a longer dwell time per MOC image line on slopes. Owing to the limitations described above, (which improves signal to noise and thus image our investigation of all of these subjects is incomplete quality), multiple-samples at a given dwell time and awaits acquisition of additional images. (increasing the spatial sampling in the down-track Landing Sites. After landing, both MER sites direction), or both. Operationally, we command the were imaged and showed the locations of the lander, spacecraft to dwell 6 times longer than normal over heat shield, and parachute/backshell. Bounce marks the target, dividing this between sampling 3 times as were evident at the MER-A site, and rocket blast the spacecraft covers a distance of 1.5 meters results were seen at the MER-B site. As the MER (downtrack) on the ground, and increasing the missions were extended, MOC cPROTO image amount of time each sample represents by a factor of mosaics became critical for long-range traverse 2 (increasing image quality by 40%). The result is a planning. Later cPROTO images documented the sharper image with ~50 cm/pixel downtrack and ~1.5 rovers’ tracks. Other PROTO and cPROTO efforts m/pixel crosstrack. The “c” in “cPROTO” is for imaged the locations of the Viking and Mars planetary motion compensation. While MGS is Pathfinder landing sites. Two attempts were made to Lunar and Planetary Science XXXVI (2005) 1172.pdf

image a candidate 2 site; the second attempt images were acquired to provide insights regarding showed that the candidate was a small impact crater these landforms. Individual mounds and hills within with—unusual for that part of —a small the textured, lineated valley floor areas exhibit dark dune in it. As of 7 January 2005, no additional smooth surfaces at the limit of cPROTO resolution Beagle 2 candidate sites have been identified. (Fig. 1d). No debris/talus is evident in the cPROTO images of candidate locations for the Mars depressions between the mounds, suggesting that if Polar Lander are scheduled to be attempted in 2005. their genesis involves erosion, the materials are Gullies. In 2004, there was a narrow period (May quickly transported away. through July) when cPROTO acquisition limitations North Polar Residual Cap and Polar Layers. permitted imaging of mid-latitude gullies. A Northern summer arrived during 2004, permitting goal of the cPROTO campaign is to test hypotheses sub-meter imaging of north polar targets. The about the genesis and evolution of the types of gullies residual ice cap surfaces are pitted (Fig. 1e); the first described by Malin and Edgett [2]. In particular, floors of pits are darker than their walls and the the presence or absence of boulders transported surfaces outside the pits, suggesting that the pits are through gully channels and deposited in their trapping non-volatile material (e.g., dust). North depositional aprons provides constraints regarding polar layers exposed in the arcuate scarp in the rheologic properties of the transporting fluids. No Boreale are distinctly grouped into 3 units, the middle boulders have yet been observed in the 8 cPROTO of which appears to be a source for dark sand. Talus images that include a gully apron area (Fig. 1b). shed from this scarp includes several large blocks Sedimentary Rocks. Sedimentary rock outcrops (boulders). are common all across the surface of Mars [3]. Eolian Features. No specific eolian targets were cPROTO images of sedimentary rock sites often selected in 2003–2004, but many cPROTO images reveal more and thinner layers than evident at lower show large, ripple-like bedforms, and one north polar spatial resolution, and they show something that was image includes sand dunes. Large ripples analogous not clearly resolved in previous MOC images—the to terrestrial granule ripples formed by grains shedding of boulders from steep slopes at transported by traction occur on the dunes in the sedimentary rock outcrop sites (Fig. 1c). The north polar image (Fig. 1f). occurrence and small size of the boulders provides References: [1] Malin M. C. (1992) JGR, 97, insights into the physical properties of the 7699–7718. [2] Malin M. C. and Edgett K. S. (2000) sedimentary rocks. Science, 288, 2330–2335. [3] Malin M. C. and Edgett Fretted Terrain Valley Floors. Fretted terrain K. S. (2000) Science, 290, 1927–1937. [4] Malin valley floors exhibit odd textures resembling M. C. and Edgett K. S. (2001) JGR, 106, corncobs and brains [4]. Half a dozen cPROTO 23429–23570.

Fig. 1. (A) MER-B Rover (from MOC image R16-02188). (B) Portion of a gully apron (R17-01367). (C) Boulders (arrows) shed from sedimentary rock outcrop in southwest (R18-01711). (D) Fretted terrain valley floor material (R19-01803). (E) North polar residual cap (R23-00003). (F) Dunes and “granule” ripples (arrows) (R23-00143). All images shown here are sub-frames of the MOC image noted. Images re-sized to fit this page.